Ignore unmatched :ignore attributes

src/AngleSharp.Diffing.Tests/DiffBuilderTest.cs

@@ -116,4 +116,17 @@ public void Test006(string control, string test)
 
         diffs.ShouldBeEmpty();
     }
+
+    [Theory(DisplayName = "When a control element has 'class:ignore', elements with and without class should return empty diffs")]
+    [InlineData("<div class=\"ian-fleming\"></div>")]
+    [InlineData("<div class=\"\"></div>")]
+    [InlineData("<div class></div>")]
+    [InlineData("<div></div>")]
+    public void Test007(string testHtml)
+    {
+        var controlHtml = "<div class:ignore></div>";
+        var diffs = DiffBuilder.Compare(controlHtml).WithTest(testHtml).Build();
+        Assert.Empty(diffs);
+    }
+
 }

src/AngleSharp.Diffing/Core/HtmlDifferenceEngine.cs

@@ -1,4 +1,5 @@
 using AngleSharp.Diffing.Core.Diffs;
+using AngleSharp.Diffing.Strategies.AttributeStrategies;
 
 namespace AngleSharp.Diffing.Core;
 
@@ -160,7 +161,13 @@ void MarkSelectedSourcesAsMatched(in AttributeComparison comparison)
 
         void UpdateUnmatchedTracking()
         {
-            Context.MissingAttributeSources.AddRange(controls.GetUnmatched());
+            // Filter out unmatched :ignore attributes, they were meant to be ignored after all
+            // https://github.com/AngleSharp/AngleSharp.Diffing/issues/48
+            var controlsUnmatched = controls
+                .GetUnmatched()
+                .Where(c => !IgnoreAttributeComparer.IsIgnoreAttribute(c.Attribute));
+
+            Context.MissingAttributeSources.AddRange(controlsUnmatched);
             Context.UnexpectedAttributeSources.AddRange(tests.GetUnmatched());
         }
     }

src/AngleSharp.Diffing/Core/SourceMap.cs

@@ -70,7 +70,7 @@ public IEnumerable<AttributeComparisonSource> GetUnmatched()
     {
         foreach (var source in _sources.Values)
         {
-            if (!_matched.Contains(source.Attribute.Name))
+            if (IsUnmatched(source.Attribute.Name))
                 yield return source;
         }
     }

src/AngleSharp.Diffing/Strategies/AttributeStrategies/IgnoreAttributeComparer.cs

@@ -15,8 +15,13 @@ public static CompareResult Compare(in AttributeComparison comparison, CompareRe
         if (currentDecision.IsSameOrSkip)
             return currentDecision;
 
-        return comparison.Control.Attribute.Name.EndsWith(DIFF_IGNORE_POSTFIX, StringComparison.OrdinalIgnoreCase)
+        return IsIgnoreAttribute(comparison.Control.Attribute)
             ? CompareResult.Same
             : currentDecision;
     }
+
+    public static bool IsIgnoreAttribute(IAttr source)
+    {
+        return source.Name.EndsWith(DIFF_IGNORE_POSTFIX,  StringComparison.OrdinalIgnoreCase);
+    }
 }